Defence to Blackwood Interference

Any of a variety of methods to cope with enemy interference over Blackwood (or Gerber.) Each is commonly known by its acronym, e.g. DOPI, ROPI, PODI, etc. Using DOPI and as examples:

Double for suit bid interference, or
Redouble for double interference
shows zero aces
Pass shows one ace
Cheapest bid shows two aces
Next bid shows three aces, etc.

Note: DOPI stands for "Double-Zero, Pass-One", and ROPI stands for "Redouble-Zero, Pass-One".